This invention relates to solid-state lasers and more particularly to solid-state lasers that employ self-pumping from internal laser transitions to achieve inversion in other transitions.
In the cascade process, laser transitions between levels in the upper manifolds are used to achieve inversion in lower-lying manifolds with resultant multiwavelength sequential emission. A number of cascade schemes have been observed in noble-gas lasers, and the cascade process is currently one of the more attractive method of obtaining 16 .mu.m radiation from CO.sub.2 lasers.
In an article in Proc. IEEE, 1974, pp. 17-31, Krupke performed calculations of laser transitions in trivalent Nd- and Dy- doped solids and discussed the possibility of cascade laser action in the mid-IR range. Experimental work on multiwavelength emission in solids via the cascade technique has been reported in Soviet Journal Quantum Electronics 5, 117, 1975; and by M. V. Petrov and A. M. Tkachuk, Optical Spectros (USSR) 45, 81, 1978. Laser action was observed between .sup.4 S.sub.3/2 .sup.4 I.sub.9/2 ; .sup.4 S.sub.3/2 .fwdarw..sup.4 I.sub.11/2 ; and .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 transitions at 1.75, 1.23, and 2.87 .mu.m, respectively. This work was limited to wavelengths shorter than 3 .mu.m. Furthermore the multiwavelength emission was observed with flashlamp pumping where it is difficult to unambiguously identify the degree of participation of the cascade process.
The longest wavelength previously observed for a rare-earth-doped laser was reported by Johnson and Guggenheim in Applied Physics Letters (23) 96, 1973. They obtained laser oscillation with Dy.sup.3+ in BaY.sub.2 F.sub.8 at 77.degree. K. The .sup.6 H.sub.13/2 .sup.6 H.sub.15/2 transition at 3.02 .mu.m had a threshold of 510J when pumped with a helical flashlamp. In their work they discuss the dominance of nonradiative decay rates due to multiphonon emission for transitions below 3000 cm.sup.-1 (.about.3.3 .mu.m). Fluorescence at these long wavelengths is overwhelmed by nonradiative relaxation, and Johnson and Guggenheim conclude that laser emission at these long wavelengths is unlikely. In the prior art it has been assumed that the upper laser level had to have a long fluorescent lifetime with a small nonradiative decay rate.